Data export/import is the only way in Oracle 12c to change the characterset from AL32UTF8 to WE8ISO8859P1

SQL>  select property_value from database_properties where property_name='NLS_CHARACTERSET';

PROPERTY_VALUE
--------------------------------------------------------------------------------
AL32UTF8

SQL> select value from nls_database_parameters where parameter='NLS_CHARACTERSET';

VALUE
--------------------------------------------------------------------------------
AL32UTF8

SQL> alter database character set INTERNAL_CONVERT WE8ISO8859P1;
alter database character set INTERNAL_CONVERT WE8ISO8859P1
*
ERROR at line 1:
ORA-12712: new character set must be a superset of old character set